Spectacles are the traditional way of correcting defective vision in the human eye. However, refractive surgery which corrects defective vision by altering the cornea is now also increasingly being used. The aim of the operating methods is to selectively alter the cornea so as to influence refraction. Differing operating methods are known for this purpose. Currently the most widespread is the so-called laser-assisted in situ keratomileusis, also abbreviated to LASIK. Firstly, a lamella of the cornea is detached on one side from the cornea surface and folded to the side. This lamella can be detached by use of a mechanical microkeratome or also by use of a so-called laser keratome, such as is marketed e.g. by Intralase Corp., Irvine, USA. After the lamella has been detached and folded to the side, the use of an excimer laser, which removes the thus-exposed corneal tissue by ablation, is envisaged in the LASIK operation. After volume in the cornea has been vaporized in this manner the lamella of the cornea is folded back into its original place.
The use of a laser keratome to expose the lamella is advantageous as the danger of infection is thereby reduced and the cut quality increased. In particular the lamella can be produced with a very much more consistent thickness. The cut is also potentially smoother, which reduces subsequent sight problems due to this boundary surface which remains even after the operation.
To produce the cut, a series of optical perforations are made at predetermined points such that the cutting surface is formed as a result. With the laser keratome the cutting surface forms the lamella to be folded back before the use of laser ablation.
With the conventional LASIK method exposed corneal tissue is vaporized, which is also called “grinding” of the cornea by means of laser radiation. The volume removal which is necessary to correct defective vision is set for each surface element of the exposed cornea by the number of laser pulses and their energy. Therefore, in the LASIK method, a so-called shot file is provided for the ablation laser, which fixes, for different points on the cornea, how often, and with what energy, the laser beam is to be directed onto defined points on the cornea. The volume removal is heuristically determined, not least because it depends greatly on the ablation action of the laser beam, therefore on the wavelength, fluence etc. of the radiation used. The state of the cornea also plays a role; in particular the moisture content of the cornea is to be mentioned here. WO 96/11655 describes a device and a process for the LASIK method. In particular a formula is given which calculates the radius of curvature of the cornea to be achieved from the pre-operative radius of curvature of the cornea and the desired dioptre correction. A similar calculation is described in EP 1153584 A1—also for corneal ablation by means of LASIK.
A further laser-based eye-surgery method is not to vaporize the volume to be removed from the cornea, but to isolate it by a laser cut. The volume is thus no longer ablated, but isolated in the cornea by a three-dimensional cutting surface and thus made removable. Empirical values which have been developed for grinding the cornea by use of ablation laser radiation cannot be used for such methods. Instead, control data are required to operate the laser for isolating the volume to be removed from the cornea. One such eye-surgery method is described in U.S. Pat. Nos. 6,110,166 and 7,131,968. Different volume forms are shown in U.S. Pat. No. 6,110,166 and it is mentioned that the proper volume can be chosen by a person skilled in the art.
DE 102006053118 A1 describes the production of control data for the volume-isolating correction of defective vision.
It is known from DE 102006053120 A1 and DE 102006053119 A1 from Carl Zeiss Meditec AG to base the production of such control data on defective vision data which give the refractive power of spectacles suitable for correcting defective vision. It is also known from this published document, which thus describes a method according to the preamble and a device according to the preamble, to use data which also bring about a correction of an astigmatism or corrections of higher-order aberrations.
The precision with which the necessary cutting surfaces are produced is of great importance for volume-isolating correction of defective vision. Unlike with a laser keratome, the position of the cutting surfaces has a direct effect on the quality of the optical correction. With the conventional LASIK method, on the other hand, the precision with which the laser ablation is carried out is the only important factor determining the quality of the optical correction. This can already be seen from the fact that the cornea lamella is or has been produced in a large number of operations with a relatively crudely operating mechanical knife.
As the exact positioning of the eye is important for the precision production of the cutting surfaces, the state of the art, for example WO 2005/011547 A1, proposes that a contact lens, against which the cornea is pressed, can be used in laser-surgery devices. This contact lens serves to fix the eye.
However, the precise position of the eye is not the only important factor for the precision of the cutting surfaces; the shape of the cornea must also be known. As this varies from patient to patient within specific ranges, the contact lens also serves to give the cornea front surface a fixed, defined shape. When pressing the front of the cornea against the contact lens, there is consequently a deformation of the cornea which varies in size depending on the deviation of the curvature of the contact lens from the natural curvature of the cornea of the respective patient.
If the position of the cutting surfaces is important for the optical correction, i.e. if not just a lamella is isolated and the volume to be removed is removed by ablation, the deformation of the cornea is essential when determining the target coordinates for producing the cutting surfaces. Therefore it is known in the state of the art to take into account the deformation by subjecting the previously determined target points to a coordinate transformation. In the named WO publication, this transformation is called a “contact pressure transformation” and there are transformation equations for a combination of spherical contact lens and spherical cornea front surface. DE 102008017293 A1 from Carl Zeiss Meditec AG adds to these transformation equations with the result that coordinate transformation can also be carried out on different types of contact lens and cornea curvatures.
The invention thus relates to the concept of carrying out a correction of the optical imaging errors of the human eye by separating, by means of laser radiation within the cornea, a volume of tissue which is then removed from the cornea. A selective change of the refractive power of the cornea is thereby achieved. This change is localized, i.e. in the area of the cornea from which the tissue volume is removed. The pupil of the eye is usually taken as a basis.
The removal of the separated volume changes the geometry, i.e. the curvature of the cornea surface. In order that a desired correction of defective vision is achieved, the separated volume for removal must therefore have special properties with regard to its shape.
As already explained, for the eye-surgical correction of defective vision the curvature which the front surface of the cornea has after correction is decisive for the corrective effect. This surface shape therefore needs to be taken into consideration during the correction of defective vision isolating the volume. In the past, as already mentioned, the main focus was on a spherical or cylindrical correction. As the corresponding change in the front surface must take place on the non-deformed eye, it was particularly advantageous that these correction surfaces could be described analytically. In this case the transformation to be brought about by pressing on the contact lens could also be described analytically.
The advantage of analytical describability is particularly evident when the corresponding target points for the laser radiation are fixed and a path curve is defined, which connects the target points and along which the focus of a treatment lens radiation has to be shifted.